Monolithic tunable terahertz radiation source using nonlinear frequency mixing in quantum cascade lasers

ABSTRACT

A terahertz difference-frequency generation quantum cascade laser source that provides monolithic, electrically-controlled tunable terahertz emission. The quantum cascade laser includes a substrate, a lower cladding layer positioned above the substrate and an active region layer with optical nonlinearity positioned on the lower cladding layer. The active region layer is arranged as a multiple quantum well structure. One or more feedback gratings are etched into spatially separated sections of the cladding layer positioned on either side of the active region. The periodicity of each grating section determines the mid-infrared lasing frequencies. The grating sections are electrically isolated from one another and biased independently. Tuning is achieved by changing a refractive index of one or all of the grating sections via a DC current bias thereby causing a shift in the mid-infrared lasing frequency. In this manner, a monolithic, electrically-pumped, tunable THz source is achieved.

RELATED APPLICATIONS

This application claims priority, under 35 U.S.C. 371, to Internationalpatent application PCT/US15/14371, “Method and Apparatus for aMonolithic Tunable Terahertz Radiation Source Using Nonlinear FrequencyMixing in Quantum Cascade Lasers,” filed Feb. 4, 2015, which claimspriority to,

U.S. Provisional Patent Application Ser. No. 61/935,400, “Method andApparatus for a Monolithic Tunable Terahertz Radiation Source UsingNonlinear Frequency Mixing in Quantum Cascade Lasers,” filed Feb. 4,2014,

Both of which are incorporated by reference herein in their entirety.

GOVERNMENT INTERESTS

This invention was made with government support under Grant nos.ECCS1150449 and ECCS0925217 awarded by the National Science Foundationand Grant no. N66001-12-1-4241 awarded by the Space and Naval WarfareSystems Center (SSC) Pacific. The government has certain rights in theinvention.

BACKGROUND

The present invention relates generally to tunable terahertz quantumcascade lasers, and more particularly to a monolithic tunable terahertzradiation source using nonlinear frequency mixing in quantum cascadelasers.

Mass-producible semiconductor sources of tunable coherent terahertz(THz) radiation in the 1-5 THz spectral range are highly desired forsensing, spectroscopy and imaging applications. Besides p-dopedGermanium lasers that require strong magnetic fields and low-temperaturecryogenic cooling for operation, quantum cascade lasers (QCLs) are theonly electrically-pumped semiconductor sources that demonstrateoperation in this entire spectral range. Narrowband THz emission hasbeen demonstrated in both THz QCLs and THz sources based on intracavitydifference-frequency generation (DFG) in mid-infrared QCLs (THzDFG-QCLs). The latter is the only technology that results inelectrically-pumped monolithic semiconductor sources operable atroom-temperature in the entire 1-5 THz range.

Single-frequency operation with wide continuous tunability is anessential requirement for THz sources for many sensing and spectroscopyapplications. Spectral tuning of THz DFG-QCLs from 1.25 to 5.9 THz hasrecently been achieved using a diffraction grating in an external cavitysetup. However, external cavity tunable laser systems are bulky, havemoving parts, and require precise alignment of optical components.Monolithic (i.e., no moving parts or external components required)electrically-tunable THz sources would be better suited for manyapplications owing to their compactness, propensity for mass-production,and high reliability due to the lack of mechanical components.

The tuning range of monolithic single-mode THz QCLs and THz DFG-QCLsources demonstrated so far is limited to below 30 GHz. Hence, there isnot a means for designing monolithic THz DFG-QCL tuners that do not haveany moving parts and can be electrically tuned over a wide tuning range.

BRIEF SUMMARY

In one embodiment of the present invention, a terahertzdifference-frequency generation quantum cascade laser source comprises aquantum cascade laser comprising a substrate. The quantum cascade laserfurther comprises a lower cladding semiconducting layer positioned abovethe substrate. The quantum cascade laser additionally comprises anactive region layer with optical nonlinearity, where the active regionlayer is positioned on the lower cladding semiconductor layer, and wherethe active region layer is arranged as a multiple quantum well structurewith optical nonlinearity for terahertz generation. Furthermore, thequantum cascade laser comprises an upper cladding semiconducting layerpositioned on the active region layer. Additionally, the quantum cascadelaser comprises two or more mid-infrared feedback gratings etched intospatially separated sections of the lower or upper claddingsemiconducting layers, where the two or more mid-infrared feedbackgratings are positioned along a length of a laser cavity, and wheremid-infrared lasing frequencies are determined by a periodicity of thetwo or more mid-infrared feedback gratings. The two or more mid-infraredfeedback gratings are electrically isolated from one another and arebiased independently to turn on or off the mid-infrared lasing.Furthermore, tuning is achieved by changing a refractive index of one orall of the two or more mid-infrared feedback gratings via a DC currentbias thereby causing a shift in a mid-infrared lasing frequency, where achange in the mid-infrared lasing frequency translates to turning ofterahertz radiation. The quantum cascade laser generates terahertzradiation via infrared difference-frequency generation andsimultaneously operates at multiple mid-infrared frequencies.Additionally, the quantum cascade laser source is designed with a modalphase matching scheme or a Cherenkov phase matching scheme to extractterahertz radiation.

The foregoing has outlined rather generally the features and technicaladvantages of one or more embodiments of the present invention in orderthat the detailed description of the present invention that follows maybe better understood. Additional features and advantages of the presentinvention will be described hereinafter which may form the subject ofthe claims of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description is considered in conjunction with thefollowing drawings, in which:

FIG. 1A illustrates a schematic of a Cherenkov THz DFG-QCL source inaccordance with an embodiment of the present invention;

FIG. 1B is a graph of the room temperature emission spectrum (blue) fora 2.7 mm cavity length device in accordance with an embodiment of thepresent invention;

FIG. 1C illustrates a waveguide cross-section for Cherenkov DFG-QCLlasers in accordance with an embodiment of the present invention;

FIG. 2A illustrates the device configuration for low-frequency mid-IRpump tuning as well as the dual-color emission spectra for different DCbias currents applied to the back section in accordance with anembodiment of the present invention;

FIG. 2B illustrates the device configuration for high-frequency mid-IRpump tuning, where the back section is unbiased while the front sectionis biased through a bias tree with both variable DC current (0 mA-300mA) and 1.3×I_(th) (2.4 A) current pulses in accordance with anembodiment of the present invention;

FIG. 3A shows the details on the tuning behavior of the two mid-IR pumpfrequencies as a function of dissipated DC power, calculated asI_(DC)×V_(DC), where I_(DC) and V_(DC) are the values of DC current andvoltage applied to the grating sections in accordance with an embodimentof the present invention;

FIG. 3B shows the details on the tuning behavior of the two mid-IR pumpfrequencies as a function of dissipated DC power, calculated asI_(DC)×V_(DC), where I_(DC) and V_(DC) are the values of DC current andvoltage applied to the grating sections in accordance with an embodimentof the present invention;

FIG. 4A illustrates the spectra of tunable THz emission measured fromthe laser in accordance with an embodiment of the present invention;

FIG. 4B illustrates the details of the tuning behavior of THz emissionfrequency in accordance with an embodiment of the present invention;

FIG. 5A illustrates the light output-current and current-voltagecharacteristics of the mid-IR pumps of the device of the presentinvention measured without any DC bias in accordance with an embodimentof the present invention;

FIG. 5B illustrates the peak THz power and mid-IR-to-THz conversionefficiency measured under the same operating conditions as in FIG. 5A inaccordance with an embodiment of the present invention; and

FIG. 6 depicts the quantum cascade laser of FIG. 1 being modified byincluding an independently controlled tuning element positioned on eachgrating section in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

In the following description, various embodiments are described. Forpurposes of explanation, specific configurations and details are setforth in order to provide a thorough understanding of the embodiments.Well-known features may be omitted or simplified in order not to obscurethe embodiment being described.

THz tuning in the difference-frequency generation (DFG) processω_(THz)=ω₁−ω₂, where ω₁>ω₂, can be achieved by changing mid-infrared(mid-IR) pump frequencies, ω₁ or ω₂. Since a small fractional shift inmid-IR pump frequency translates into a large fractional change of THzemission frequency, this approach leads to monolithic THz semiconductorsources with an extremely wide tuning range as discussed further below.To independently control two mid-IR pump frequencies, the device of thepresent invention includes two independently-biased distributed gratingsections for each mid-infrared pump wavelength. By controlling the DCcurrent through these sections, one can electrically tune ω₁ or ω₂ viathermally changing the refractive index of the section. The mid-IR pumpfrequencies in the devices of the present invention can only be redshifted with an increase of DC current; however, THz emission frequencyis given by the difference of the two mid-IR frequencies and thus can beboth blue and red shifted depending on the choice of the mid-IRfrequency to tune as discussed further below. The operating principle ofsuch THz sources is depicted in FIGS. 1A-1C.

FIG. 1A illustrates a schematic of a Cherenkov THz DFG-QCL source inaccordance with an embodiment of the present invention. FIG. 1B is agraph of the room temperature emission spectrum (blue) for a 2.7 mmcavity length device. FIG. 1C illustrates a waveguide cross-section forCherenkov DFG-QCL lasers in accordance with an embodiment of the presentinvention.

Referring to FIGS. 1A-1C, a broadband THz DFG-QCL source includes aquantum cascade laser 100, which includes a substrate 101 that may becomprised of a III-V semiconductor compound, such as InP. In oneembodiment, substrate 101 is formed of semi-insulating, undoped or verylow doped (concentration of dopant <10¹⁶ cm⁻³) indium phosphide. In oneembodiment, substrate 101 has a thickness between 100 μm and 3,000 μm.In another embodiment, substrate 101 has a thickness of less than 100 μmor more than 3,000 μm.

Furthermore, quantum cascade laser 100 includes a doped currentextraction semiconductor layer 102 positioned on substrate 101.Furthermore, quantum cascade laser 100 includes an active region layer103 surrounded by waveguide semiconducting clad layers 104, 105 (cladlayer 104 is identified as “up clad” in FIG. 1A and clad layer 105 isidentified as “low clad” in FIG. 1A), where clad layer 105 is positionedon top of current extraction semiconductor layer 102. As will bediscussed further herein, current extraction layer semiconductor layer102 is used for lateral current extraction from active region layer 103in the Cherenkov waveguide configuration. In one embodiment, currentextraction layer 102 and waveguide clad layer(s) 105 are the same layer.Waveguide clad layers 104, 105 are disposed to form a waveguidestructure to guide mid-infrared light by which terahertz radiationgenerated in active region layer 102 is emitted by laser 100.Additionally, a contact layer 106 is formed on top of the upper side ofwaveguide clad layer(s) 104 as shown in FIG. 1C. Furthermore, aninsulation layer 107, such as Si_(x)N_(y) (e.g., Si₃N₄), is depositedover contact layer 106, cladding layers 104, 105 and active region 103as illustrated in FIG. 1C. In another embodiment of the presentinvention, the silicon nitride of insulation layer 107 is replaced bysemi-insulating InP to form a buried heterostructure waveguide.Additionally, contact layer 108 is formed on top of contact layer 106and insulation layer 107 as illustrated in FIG. 1C.

Active region layer 102 includes semiconductor layers that generatelight of a predetermined wavelength (for example, light in themid-infrared wavelength range) and provide giant optical nonlinearityfor terahertz difference-frequency generation by making use ofintersubband transitions in a quantum well structure. In the presentembodiment, in correspondence to the use of an InP substrate 101 as thesemiconductor substrate, active region layer 102 is arranged as anInGaAs/InAlAs multiple quantum well structure that uses InGaAs inquantum well layers and uses InAlAs in quantum barrier layers.

Specifically, active region layer 102 is formed by multiple repetitionsof a quantum cascade structure in which the light emitting layers andelectron injection layers are laminated. The number of quantum cascadestructure repetitions in the active region is set suitably and is, forexample, approximately 10-80 for mid-infrared QCLs and THz DFG-QCLs.

In one embodiment, active region layer 102 includes one or moredifferent quantum cascade sections designed for a broad mid-IR spectralgain bandwidth spanning anywhere from 0.1 THz-10 THz.

Two mid-IR pumps at frequencies ω₁ and ω₂ propagate in the laserwaveguide with active region 102 designed to possess giant second-ordernonlinearity χ⁽²⁾ for terahertz DFG. The laser waveguide is designed sothat the THz frequency generated via the DFG process in the QCL activeregion is emitted into the InP device substrate 101 at a “Cherenkov”angle β_(c) given as:

$\begin{matrix}{{\cos \left( \beta_{c} \right)} = {\frac{\beta_{1} - \beta_{2}}{k_{THz}} = \frac{n_{g}}{n_{sub}}}} & (1)\end{matrix}$

where β₁ and β₂ are the propagation constants of the two mid-IR pumps,k_(THz) is the k-vector of the terahertz wave at frequency ω_(THz)=ω₁−ω₂in the substrate, n_(g) is the group refractive index of the mid-IR pumpmodes, and n_(sub) the refractive index of the substrate at ω_(THz).

Furthermore, as illustrated in FIG. 1A, quantum cascade laser 100includes two grating sections 109A, 109B etched into separate sectionsof clad layer 104 and covered by metal 110. In one embodiment, gratingsections 109A, 109B may be etched into separate sections of clad layer105. In one embodiment, grating sections 109A, 109B are positioned alonga length of the laser cavity 111 of laser 100 as showing FIG. 1A. In oneembodiment, grating section 109A is designed to select a high (ω₁)mid-IR pump frequency and grating section 109B is designed to select alow (ω₂) pump frequency. Each grating section 109A, 109B in FIG. 1A canbe independently biased to turn on or off the mid-infrared lasing and isseparated by a gap etched through the heavily-doped top waveguide layer104 to avoid electrical cross-talk (i.e., electrically isolated from oneanother) as discussed further below. In one embodiment, the length ofgrating sections 109A, 109B is approximately 0.05 mm to 50 mm. In oneembodiment, the length of the gap between gating sections 109A, 109B isapproximately 5 μm to 5,000 μm. Grating sections 109A, 109B maycollectively or individually be referred to as grating sections 109 orgrating section 109, respectively. While FIG. 1A illustrates two gratingsections 109, quantum cascade laser 100 may include additional gratingsections 109. The description herein regarding grating sections 109A,109B applies to each of these additional grating sections.

The grating periods were selected to position the two mid-IR pumpwavelengths as shown in FIG. 1B. That is, the periodicity of gratings109 is used to determine the mid-infrared lasing frequencies. Thefrequency separation between ω₁ and ω₂ was chosen to provide THzemission at 3.5 THz, where the best performance of DFG-QCLs is currentlyachieved. In one embodiment, 2.7-mm-long ridge waveguide devices werefabricated with a 22 μm-wide-ridge widths. The lasers had two 1.2mm-long grating sections separated by a 300 μm gap. Details ofprocessing steps are discussed further below.

The lasers were operated by applying pulsed current above a lasingthreshold to front section 109A. In this configuration, the grating inthe front section 109A operates as distributed feedback grating (DFB),while the grating in the back section 109B operates as distributed Braggreflector grating (DBR), as shown in FIG. 1A. In one embodiment,wavelength tuning is achieved by applying a DC bias below the lasingthreshold either to back grating section 109B or to front gratingsection 109A. In the latter case, the DC bias was supplied through abias tee. It is noted that while temperature tuning is employed tochange mid-IR pump frequencies, other tuning mechanisms demonstrated inmid-IR QCLs, such as voltage tuning or optical tuning, may be employedas well.

Initial device testing was performed by applying pulsed current to frontsection 109A only without using any DC bias. Dual-color single-modeemission with 1/λ₁₌₁₀₅₆ cm⁻¹ and 1/λ₂₌₉₃₇ cm⁻¹ was observed for pumpcurrents up to 1.6×I_(th) (1.6×threshold current), in excellentagreement with the grating design. At pump currents above 1.6×I_(th),additional lasing modes appeared close to the center of the gain. Thewavelength tuning performance of the device of the present invention wasinvestigated at pulsed pump current of 1.3×I_(th) applied to frontsection 109A, well within the dynamic range of the single-mode pumpsoperation.

Wavelength tuning was achieved by applying DC bias either to the frontor to the back section 109A, 109B, respectively. The tuning rate isexpected to be proportional to the temperature change in the lasersections, which is in turn proportional to the dissipated electricalpower. FIG. 2A illustrates the device configuration for low-frequencymid-IR pump tuning as well as the dual-color emission spectra fordifferent DC bias currents applied to back section 109B in accordancewith an embodiment of the present invention. FIG. 2B illustrates thedevice configuration for high-frequency mid-IR pump tuning, where backsection 109B is unbiased while front section 109A is biased through abias tree with both variable DC current (0 mA˜300 mA) and 1.3×I_(th)(2.4 A) current pulses in accordance with an embodiment of the presentinvention.

Referring to FIGS. 2A-2B, FIGS. 2A-2B show the tuning of mid-IR emissionspectra as a function of DC current applied to laser sections 109A-109B.FIG. 2A displays the results when the DC bias is applied to back section109B of the laser. As expected, the low frequency pump ω₂ showssignificant red-shift due to increase of the effective modal refractiveindex in DBR section 109B with bias current. FIG. 2B displays the tuningof mid-IR pumps when DC bias is applied to front section 109A of thelaser. In this case, the high frequency ω₁ shows significant red-shift.

FIGS. 3A-3B show the details on the tuning behavior of the two mid-IRpump frequencies as a function of dissipated DC power, calculated asI_(DC)×V_(DC), where I_(DC) and V_(DC) are the values of DC current andvoltage applied to laser sections 109A-109B in accordance with anembodiment of the present invention. Elements 301 indicate the spectralpositions of the measured mid-IR peaks. Lines 302 show the calculatedposition of the DFB mode (left panels) and the DBR reflection bandwidth(right panels) as a function of dissipated power. Lines 303 in bothright panels indicate the mid-point of the DBR bandwidth. Lines 304 inthe right panels in FIGS. 3A and 3B show the calculated laser cavitymodes for DBR lasing as a function of DC bias currents.

Referring to FIGS. 3A-3B, as expected, the tuning rate is linearlyproportional to the dissipated power applied to the tuning section. Thespectral position of the high-frequency mid-IR mode w, is determined bythe DFB grating in the laser cavity and it changes continuously withtemperature. Over 6 cm⁻¹ (0.2 THz) of continuous w, tuning is observedwhen the DC bias is applied to front section 109A of the laser as shownin FIG. 3A. When the DC bias is applied to back section 109B of thedevice, very small tuning of ω₁ is still observed due to heat spreadingto front DFB section 109A of the device (see FIG. 3B). The evolution ofthe spectral position of the low-frequency mid-IR mode is morecomplicated. Principally, it is determined by the position of the lasercavity modes within the high reflectivity band of the tunable DBRmirror, cf. FIG. 1A. The mid-IR pump ω₂ shows continuous tuning forapproximately 0.5 cm⁻¹ and mode hopping to the next laser cavity modespaced by approximately 0.9 cm⁻¹. This behavior can be well-explained bycalculating the effective laser cavity length for the DBR mode ofLDBR≈1.7 mm that gives mode spacing of 0.88 cm⁻¹ (26 GHz). Thecalculated dependence of the spectral positions of the DBR laser cavitymodes as a function of DBR or DFB bias are shown as lines 304 in FIGS.3A-3B. Details of these calculations are provided further below. Over 16cm⁻¹ (0.4 THz) of ω₂ tuning is achieved when the DC bias is applied toback section 109B of the device as shown in FIG. 3B. When the bias isapplied to front section 109A, the ω₂ pump mode shows zigzag tuningpattern as the effective laser cavity length changes (see FIG. 3A).

Spectra of tunable THz emission measured from the laser are shown inFIG. 4A in accordance with an embodiment of the present invention. FIG.4A illustrates the THz spectra for various DC biases applied to DBRsection 109B (line 401) or DFB section 109A (line 402). THz emissionspectrum from a device without applying a DC bias is shown in line 403.The top inset of FIG. 4A illustrates the fine tuning of THz emissionaround the mode-hop point.

Referring to FIG. 4A, the linewidth of THz emission was measured to be10 GHz in the whole tuning range, limited by the spectral resolution ofthe spectrometer (see below discussion). As the DC bias is applied toback section 109B of the laser, low frequency mid-IR pump ω2 is redshifted and the frequency separation between two mid-IR pumps increasesleading to the blue shift of the THz DFG emission. When the DC bias isapplied to front section 109A of the device, the frequency of mid-IRpump ω1 is reduced leading to the red shift of THz DFG emission. A totaltuning range of 0.58 THz or over 15% of the THz center frequency isachieved in the devices of the present invention. Details of the tuningbehavior of THz emission frequency are shown in FIG. 4B in accordancewith an embodiment of the present invention.

Referring to FIG. 4B, elements 404 indicate THz emission frequencyestimated from the peak spectral positions of the mid-IR pumpfrequencies shown in FIG. 3A. Elements 405 are the experimentallymeasured positions of THz emission frequencies as shown in FIG. 4A. Asillustrated in FIG. 4B, the measured THz emission frequencies are inperfect agreement with expectations. Continuous single-mode tuning nearthe mode-hop points is achieved by adjusting DC bias voltages to bothfront and back sections 109A-109B of the laser. Demonstration ofcontinuous tuning across the mode-hop region around 3.6 THz (see element406 in FIG. 4B) is shown in the inset of FIG. 4A. To achieve the finetuning, a second DC bias (dissipated power in the range of 60 to 250 mW)was applied to DFB section 109A to shift the DFB mode towards the longwavelength side while DBR section 109B was biased at a constant 370 mWDC power level. The THz peak power tuning curve is shown in FIG. 4B. Forpower measurements, the device was operated with 1.3×I_(th)=2.4 Acurrent pulses (50 kHz, 50 ns) applied to front DFB section 109A. TheTHz power output is slightly increased at DFB DC bias power of 500 mWdue to the associated increase of the high-frequency (ω₁) mid-IR pumpintensity and then experiences gradual drop at high DC bias as mid-IRpump powers are reduced.

Light output-current and current-voltage characteristics of the mid-IRpumps of the device of the present invention measured without any DCbias are shown in FIG. 5A in accordance with an embodiment of thepresent invention. FIG. 5B illustrates the peak THz power andmid-IR-to-THz conversion efficiency measured under the same operatingconditions as in FIG. 5A in accordance with an embodiment of the presentinvention. Referring to FIGS. 5A and 5B, elements 501, 502 and 503indicate the short wavelength pump (λ_(S)) power, the long wavelengthpump (λ_(L)) power and the applied voltage, respectively. Formeasurements shown in FIGS. 5A and 5B, the 1.2-mm-long and 22-μm-wideDFB section 109A was driven by pulse current with 50 kHz repetitionfrequency and 50 ns pulse width at 20° C., while the 0.3-mm-long gap and1.2-mm-long DBR section 109B was unbiased. Furthermore, no collectionefficiency was introduced to compensate THz power loss through theparabolic mirror setup, which leads to underestimation of THz power. Themid-IR power measurements were performed with estimated 100% collectionefficiency. Maximum THz peak power was recorded as high as 6.3 μW with amid-IR to THz nonlinear conversion efficiency of approximately 0.4 mWW⁻²near threshold and 0.2 mWW⁻² near the rollover point. The reduction ofmid-IR to THz conversion efficiency is attributed to the reduction ofoptical nonlinearity due to change of the QCL bandstructure alignment athigher bias voltages.

The tuning range of 580 GHz is believed to be the largest tuning rangeobtained from a monolithic, electrically-pumped single-mode terahertzsemiconductor source.

External cavity tuning of THz DFG-QCL chips and measurements of DFB THzDFG-QCL devices processed from the same wafer indicate that the THztuning range of monolithic DFG-QCL sources can in principle be extendedto span the entire 1-6 THz spectral range and beyond, limited only bythe transparency window of InGaAs/AlInAs/InP materials and the rolloverof THz DFG efficiency at low THz frequencies, as long as one finds a wayto monolithically tune mid-IR pump or pumps over broad spectral range.Recent demonstrations of monolithic single-mode mid-IR QCL tuners basedon sampled gratings with over 230 cm⁻¹ (nearly 7 THz) tuning rangeindicate that future monolithic THz DFG-QCL sources may achieve spectralcoverage of the entire 1-6 THz frequency window and beyond. The devicesof the present invention may also be integrated into arrays of lasers,similarly to that demonstrated in mid-IR, to provide continuous spectralcoverage over broad THz spectral range.

As a result, it has been demonstrated herein that the THz DFG-QCLtechnology may enable mass-production of broadband monolithicsemiconductor THz tuners with electrical emission frequency control. Asthe performance of THz DFG-QCL designs is being improved, compactelectrically-controlled THz DFG-QCL tuners are expected to findapplications in a wide variety of THz systems and are expected todramatically reduce their size and complexity.

In one embodiment, laser heterostructure 100 was grown on a 350 μm thicksemi-insulated InP substrate 101 using a metal organic vapor phaseepitaxy system. A 200-nm-thick InGaAs layer 102 n-doped to 1×10¹⁸ cm⁻³was grown on top of substrate 101 for lateral current extraction,followed by a 3.5-μm-thick lower InP cladding layer 105 n-doped to1.5×10¹⁶ cm⁻³, a 4.2-μm-thick active region 103 made of two QCL stacks,and a 3.5-μm-thick upper InP cladding layer n-doped to 1.5×10¹⁶ cm⁻³.The growth was finalized with a 500-nm-thick InP outer cladding layer(combined with upper InP cladding layer to form cladding layer 104 asshown in FIGS. 1A and 1C) n-doped to 3.5×10¹⁸ cm⁻³ and a 20-nm-thickInGaAs contact layer 106 n-doped to 1×10¹⁹ cm⁻³.

In one embodiment, device fabrication started with removing the InGaAscontact layer 106 and reducing the thickness of the heavily doped InPouter cladding layer 104 from 500 nm to 100 nm to enhance the couplingbetween the laser mode and top surface gratings 109A-109B.Rectangular-shaped first order gratings with 50% duty cycle have beenformed using electron-beam lithography. The length of both gratingsections 109A-109B is 1.2 mm, resulting in a total cavity length of 2.7mm including a 300 μm gap between sections 109A-109B. The 300 μm gap wasetched through the remainder of the heavily doped InP outer claddinglayer 104 to minimize electrical crosstalk between sections 109A-109B.The cross-talk resistance between grating sections 109A-109B wasmeasured to be 700Ω at room temperature. This device configurationresulted in the two mid-IR pumps providing roughly equal amount ofoptical power near the rollover point.

Top DFB/DBR grating period was chosen to be 1.65 μm for the mid-IR pumpwavelength of 10.6 μm and 1.48 μm for the mid-IR pump wavelength of 9.5μm. Gratings 109A-109B were etched to 170 nm±10 nm depth and 22-μm-wideridges with grating on top were then processed via dry etching. A400-nm-thick SiN layer was deposited conformally and opened on top ofthe ridges for electrical contact. Metal contacts 110 (Ti/Au=20 nm/400nm) for current injection and lateral extraction were then formed byevaporation and liftoff. Finally, the wafer was cleaved into 2.7-mm-longlaser bars and the front facet of substrate 101 was polished at 30degree angle for outcoupling of the Cherenkov radiation. Laser bars werethen wire-bonded and mounted on copper blocks using indium paste.

1. Experimental Measurements

All optical measurements were performed under pulsed bias current with50 kHz repetition rate and 50 ns pulse width at 20° C. Mid-IR opticalpower of each pump was measured using a calibrated thermopile detector.Optical filters were used to perform power measurements for each of thetwo mid-IR pumps. THz optical power was measured using a calibratedGolay cell detector and off-axis parabolic mirrors under N₂ purgedcondition to minimize water absorption. Mid-IR and THz spectra weremeasured using a Fourier-transform infrared spectrometer (FTIR) equippedwith a deuterated L-alanine doped triglycine sulphate (DTGS) detectorand a helium-cooled Si bolometer, respectively. The nominal FTIRspectral resolution is 0.2 cm⁻¹ for mid-IR and ˜0.25 cm⁻¹ for THzmeasurements.

The cavity mode spacing for the DBR laser is determined by the DBR lasercavity length LDBR that is made up of the length of front section 109A,the length of the gap, and the effective length of the DBR 109B (seeFIG. 1A). The effective DBR length, L_(eff), corresponds to theeffective length of optical power penetration into grating 109B and isdetermined by the coupling constant, k. Assuming the effectiverefractive index of DBR section 109B is close to the group index of theFabry-Perot (FP) QCLs, the effective grating length L_(eff) and couplingconstant k can be estimated using the relation:

$L_{eff} = {\frac{1}{2k}\left( {{{\tan h}\left( {kL}_{g} \right)},} \right.}$

where is the physical length of DBR grating 109B. Taking the value ofthe coupling constant to be 25 cm⁻¹ in accordance with simulations, oneobtains ≈200 μm and the total DBR cavity length is L_(DBR)=1.7 mm. Themodal spacing for the DBR laser can then be determined asΔ(1/λ)=1/(2n_(g)LDBR)≈0.88 cm⁻¹, where n_(g)≈3.35 was used. This resultis an excellent agreement with the experimental measurement of 0.9 cm⁻¹.

2. Temperature Increase in the Laser Sections

The laser was operated with 50 ns pulsed current and no DC bias wasapplied to any of the laser sections 109A-109B. The data in FIGS. 3A-3B(discussed above) allows one to estimate the temperature tuning rated(1/λ)/dT, in the device of the present invention to be −0.064 cm⁻¹K⁻¹for the high mid-IR pump frequency (ω₁) and −0.056 cm⁻¹K⁻¹ for the lowmid-IR pump frequency (ω₂). One can then use these coefficients todeduce the temperature change in the DFB and DBR sections 109A-109B fordifferent applied DC powers shown in FIGS. 3A-3B. The maximumbias-induced temperature increase in the DFB and DBR sections 109A-109Bis approximately 100° C. and 250° C., respectively.

3. Heat Diffusion Between the DFB and DBR Sections

FIGS. 3A-3B show the dependences of the mid-IR emission frequencies inthe device of the present invention on the DC power applied either toDFB section 109A or to DBR section 109B. Nearly linear dependence of thefrequency change on the applied DC power is observed in all cases. Inparticular, the tuning rate of the DFB mode was measured to be −2.94cm⁻¹ W⁻¹ when the DC bias is applied to DFB section 109A and still to be−0.37 cm⁻¹ W⁻¹ when the DC bias was applied to DBR section 109B. Giventhe values of d(1/λ)/dT coefficients obtained above, one obtains a rateof the average temperature increase in DFB section 109A to be 45.9 K·W⁻¹and 5.8 K·W⁻¹ when the DC power is applied to DFB section 109A and DBRsection 109B, respectively. Since the device has a symmetric geometry,the same picture applies for temperature increase in DBR section 109B.

4. Laser Tuning Characteristics

The spectral position of the DFB lasing mode is determined by the Braggwavelength of DFB grating 109A and one expects continuous tuning of theDFB lasing mode as the temperature of DFB section 109A is continuouslychanging, assuming mirror reflectivity is negligible. In contrast, thespectral position of the DBR mode is determined by the position of thelaser cavity mode closest to the DBR mirror reflectivity peak and modehopping behavior of the DBR laser emission is expected as thetemperature of DBR section 109B is changed.

The relative shift of the spectral position of the DFB mode is given as,

$\begin{matrix}{{\frac{\Delta \; v_{B - {DFB}}}{v_{B - {DFB}}} = \frac{\Delta \; n_{{eff}\_ {DFB}}}{n_{{eff}\_ {DFB}}}},} & (2)\end{matrix}$

where n_(eff) _(_) _(DEB) (Δn_(eff) _(_) _(DEB)) is the value (change invalue) of the effective refractive index of the laser mode in DFBsection 109A.

The relative frequency shift of the peak of DBR mirror reflectivity(Δv_(B-DBR)/v_(B-DBR)) and the frequency change in the cavity modeposition (Δv_(C)/v_(C)) as a function of the change of refractiveindices in different sections of our device can be expressed as,

$\begin{matrix}{{\frac{\Delta \; v_{B - {DBR}}}{v_{B - {DBR}}} = \frac{\Delta \; n_{{eff}\_ {DBR}}}{n_{{eff}\_ {DBR}}}},} & {(3),} \\{{\frac{\Delta \; v_{C}}{v_{C}} = \frac{{\Delta \; n_{{eff}\_ {DFB}}L_{DFB}} + {\Delta \; n_{{eff}\_ {gap}}L_{gap}} + {\Delta \; n_{{eff}\_ {DBR}}L_{eff}}}{\; {{n_{{eff}\_ {DFB}}L_{DFB}} + {n_{{eff}\_ {gap}}L_{gap}} + {n_{{eff}\_ {DBR}}L_{eff}}}}},} & (4)\end{matrix}$

where n_(eff) _(_) _(DBR) (Δn_(eff) _(_) _(DBR)), n_(eff) _(_) _(gap)(Δn_(eff) _(_) _(gap)), and n_(eff) _(_) _(DEB) (Δn_(eff) _(_) _(DFB))are the values (change in values) of the effective refractive indices ofthe long-wavelength laser mode ω₂ in DBR section 109B, in the gapbetween DFB and DBR sections 109A-109B, and in DFB section 109A,respectively, L_(DFB) is the length of DFB section 109A, L_(gap) is thelength of the gap between DFB and DBR sections 109A-109B, and L_(eff) isthe effective grating length for DBR section 109B defined earlier. Inthe analysis discussed herein, it was assumed that n_(eff) _(_)_(DBR)≈n_(eff) _(_) _(gap)≈n_(eff) _(_) _(DFB) for simplicity.

As DC bias on DFB section 109A increases, the effective refractiveindices in different sections of the device of the present inventionincrease due to temperature rise. The process can approximately beexpressed as,

Δn_(eff) _(_) _(DFB)≈S_(DFB) ^((DFB))P_(dis) ^((DFB)),  (5)

Δn_(eff) _(_) _(DBR)≈S_(DBR) ^((DFB))P_(dis) ^((DFB)),  (6)

Δn_(eff) _(_) _(gap)≈S_(gap) ^((DFB))P_(dis) ^((DFB)),  (7)

where P_(dis) ^((DFB)) is the dissipated power applied to DFB section109A, and S_(DFB) ^((DFB)), S_(DBR) ^((DFB)), and S_(gap) ^((DFB)) arethe effective refractive index tuning coefficients in the DFB 109A, DBR109B, and gap sections, respectively. The values of S_(DFB)^((DFB))=0.92×10⁻² W⁻¹ and S_(DBR) ^((DFB))=0.12×10⁻² W⁻¹ are determinedfrom the experimental data on modal tuning shown in FIG. 3A, using therelation:

${n_{eff} = \frac{\pi}{2\Lambda}},$

where is the grating period and λ is the emission wavelength. Equations(4), (6), and (7) are then used to plot the position of the DBR lasercavity modes in the right panel in FIG. 3A. The contribution of Δn_(eff)_(_) _(gap) was ignored in the simulation due to its relatively shortlength though it can also be used as a fitting parameter.

Similarly, as DC bias on DBR section 109B increases, the effectiverefractive indices in various sections of the device change according tothe expressions:

Δn_(eff) _(_) _(DBR)≈S_(DBR) ^((DBR))P_(dis) ^((DBR)),  (8)

Δn_(eff) _(_) _(DFB)≈S_(DFB) ^((DRB))P_(dis) ^((DBR)),  (9)

Δn_(eff) _(_) _(gap)≈S_(gap) ^((DBR))P_(dis) ^((DBR)),  (10)

where P_(dis) ^((DFB)) is the dissipated power applied to DFB section109A, and S_(DFB) ^((DFB)), S_(DBR) ^((DFB)), and S_(gap) ^((DFB)) arethe effective refractive index tuning coefficients in the DFB 109A, DBR109B, and gap sections, respectively. The values of S_(DFB)^((DBR))=0.92×10⁻² W⁻¹ and S_(DBR) ^((DBR))=0.12×10⁻² W⁻¹ are determinedfrom the experimental data on modal tuning shown in FIG. 3B as describedabove. Equations (4), (8), and (9) are then used to plot the position ofthe DBR laser cavity modes in the right panel in FIG. 3B. Thecontribution of Δn_(eff) _(_) _(gap) was ignored in the simulation forthe same reason noted above.

As a result of designing a quantum cascade laser using the principles ofthe present invention discussed above, an electrically pumped andcompletely monolithic (i.e., it requires no moving parts or externalcomponents) THz DFG-QCL tuner can be achieved. This is in contrast tocompeting semiconductor THz source technologies of similar size, such asphotomixcrs, photoconductive switches, external cavity THz QCLs andexternal cavity THz DFG-QCLs. An all-monolithic construction is cheaperto manufacture, rugged, compact, simpler to design and operate, andenables seamless integration in larger system solutions.

The present invention can operate in a spectral region (0.5-10 THz)inaccessible by electronic mixers/multipliers/photomixers (maximum 2.5THz). While photoconductive switches and optical parametric oscillators(OPOs) can operate over a wide spectral range, they are prohibitivelylarge, expensive to manufacture, complex to operate and provide onlybroadband output with limited tuning. However, the present invention isextremely compact, cost-effective, and can generate tunable,single-frequency THz radiation that is highly desired forfrequency-domain spectroscopic applications. Additionally, the presentinvention can operate at room-temperature which is a significantadvantage compared to traditional THz QCL systems or p-Ge lasers thatrequire cryogenic cooling.

An alternative embodiment of the present invention is implementing asource with two or more feedback grating sections 109 (FIG. 1A) formulti-wavelength mid-infrared lasing and multi-wavelength tunableterahertz generation.

A further embodiment of the present invention is implementing a devicethat decouples the DC current required for mid-infrared tuning from theelectrical bias required to activate/quench the lasing wavelength. Onesuch configuration is shown in FIG. 6 which depicts quantum cascadelaser 100 of FIG. 1 being modified by including an independentlycontrolled tuning element 601A-601B positioned on each grating section109A-109B, respectively, along with an insulating layer 602A-602B toseparate the DC bias sections (labeled as “DC Bias 2” and “DC Bias 1” inFIG. 6) from grating sections 109A-109B, respectively, in accordancewith an embodiment of the present invention. The quantum cascade laser(QCL) bias (labeled as “QCL Bias 1” and “QCL Bias 2”) discussed above isalso shown in FIG. 6. Tuning elements 601A-601B may collectively orindividually be referred to as tuning elements 601 or tuning element601, respectively. Tuning elements 601 can be monolithically fabricatedalongside grating elements 109, or comprise of external elements affixedto each grating section 109. Tuning elements 601 are electricallyisolated from one another and from the rest of the device. Thetemperature of each tuning element 601 can be independently controlledwith a DC current, where the DC current applied to tuning elements 601is independent of an electrical bias required to activate and quench themid-infrared lasing. Alternatively, the temperature of tuning element601 can be independently changed via optically induced heating from anexternal laser source. The change in the tuning element temperaturecauses a shift in the mid-infrared lasing wavelength and results interahertz tuning.

In another embodiment of the present invention, a tunable terahertzsource with broad spectral coverage includes an array of monolithicallytunable terahertz difference-frequency generation quantum cascadelasers. Each laser in the array operates and tunes in a specificterahertz spectral band.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

1. A method comprising: generating terahertz radiation with a quantumcascade laser via infrared difference-frequency generation, wherein thequantum cascade laser is simultaneously operating at multiplemid-infrared frequencies, wherein the quantum cascade laser is designedwith a modal phase matching scheme or a Cherenkov phase matching schemeto extract the terahertz radiation, wherein the quantum cascade lasercomprises: a substrate; a lower cladding semiconducting layer positionedabove said substrate; an active region layer with optical nonlinearity,wherein said active region layer is positioned on said lower claddingsemiconductor layer, wherein said active region layer is arranged as amultiple quantum well structure with optical nonlinearity for terahertzgeneration; an upper cladding semiconducting layer positioned on saidactive region layer; and two or more mid-infrared feedback gratingsetched into spatially separated sections of said lower or upper claddingsemiconducting layers, wherein said two or more mid-infrared feedbackgratings are positioned along a length of a laser cavity, whereinmid-infrared lasing frequencies are determined by a periodicity of saidtwo or more mid-infrared feedback gratings, wherein said two or moremid-infrared feedback gratings are electrically isolated from oneanother and are biased independently to turn on or off said mid-infraredlasing, wherein tuning is achieved by changing a refractive index of oneor all of said two or more mid-infrared feedback gratings via a DCcurrent bias thereby causing a shift in a mid-infrared lasing frequency,wherein a change in said mid-infrared lasing frequency translates totuning of terahertz radiation.
 2. The method as recited in claim 1,wherein periods of said two or more mid-infrared feedback gratingsspectrally determine mid-infrared pump wavelengths.
 3. The method asrecited in claim 1, wherein each of said two or more mid-infraredfeedback gratings is independently electrically biased to activate orquench said mid-infrared lasing.
 4. The method as recited in claim 1,wherein red or blue shifted wavelength tuning of said mid-infraredlasing frequency is controlled by an applied DC current.
 5. The methodas recited in claim 4, wherein said applied DC current is combined witha quantum cascade laser bias.
 6. The method as recited in claim 1,wherein said two or more mid-infrared feedback gratings have a length ofapproximately 0.05 mm to 50 mm.
 7. The method as recited in claim 1,wherein a gap between each of said two or more mid-infrared feedbackgratings is etched into said upper cladding semiconducting layer toelectrically isolate and minimize crosstalk between each of said two ormore mid-infrared feedback gratings.
 8. The method as recited in claim7, wherein said gap between each of said two or more mid-infraredfeedback gratings has a length of approximately 5 μm to 5,000 μm.
 9. Themethod as recited in claim 1, further comprising: tuning elementsmonolithically fabricated alongside said two or more mid-infraredfeedback gratings or comprise external elements affixed to each of saidtwo or more mid-infrared feedback gratings, wherein said tuning elementsare electrically isolated from one another, wherein a temperature ofeach of said tuning elements is independently controlled with a DCcurrent, wherein said DC current applied to said tuning elements isindependent of an electrical bias required to activate and quench saidmid-infrared lasing.
 10. The method as recited in claim 1, wherein thequantum cascade laser further comprises an array of said quantum cascadelasers, wherein each of said quantum cascade lasers is designed to emitand tune over a specific terahertz spectral range.
 11. A terahertzdifference-frequency generation quantum cascade laser source,comprising: a quantum cascade laser comprising: a substrate; a lowercladding semiconducting layer positioned above said substrate; an activeregion layer with optical nonlinearity, wherein said active region layeris 6 positioned on said lower cladding semiconductor layer, wherein saidactive region layer is arranged as a multiple quantum well structurewith optical nonlinearity for terahertz generation; an upper claddingsemiconducting layer positioned on said active region layer; and two ormore mid-infrared feedback gratings etched into spatially separatedsections of said lower or upper cladding semiconducting layers, whereinsaid two or more mid-infrared feedback gratings are positioned along alength of a laser cavity, wherein mid-infrared lasing frequencies aredetermined by a periodicity of said two or more mid-infrared feedbackgratings, wherein said two or more mid-infrared feedback gratings areelectrically isolated from one another and are biased independently toturn on or off said mid-infrared lasing, wherein tuning is achieved bychanging a refractive index of one or all of said two or moremid-infrared feedback gratings via a DC current bias thereby causing ashift in a mid-infrared lasing frequency, wherein a change in saidmid-infrared lasing frequency translates to tuning of terahertzradiation; and wherein said quantum cascade laser generates terahertzradiation via infrared difference-frequency generation andsimultaneously operates at multiple mid-infrared frequencies, whereinsaid quantum cascade laser is designed with a modal phase matchingscheme or a Cherenkov phase matching scheme to extract terahertzradiation.
 12. The terahertz difference-frequency generation quantumcascade laser source as recited in claim 11, wherein periods of said twoor more mid-infrared feedback gratings spectrally determine mid-infraredpump wavelengths.
 13. The terahertz difference-frequency generationquantum cascade laser source as recited in claim 11, wherein each ofsaid two or more mid-infrared feedback gratings is independentlyelectrically biased to activate or quench said mid-infrared lasing. 14.The terahertz difference-frequency generation quantum cascade lasersource as recited in claim 11, wherein red or blue shifted wavelengthtuning of said mid-infrared lasing frequency is controlled by an appliedDC current.
 15. The terahertz difference-frequency generation quantumcascade laser source as recited in claim 14, wherein said applied DCcurrent is combined with a quantum cascade laser bias.
 16. The terahertzdifference-frequency generation quantum cascade laser source as recitedin claim 11, wherein said two or more mid-infrared feedback gratingshave a length of approximately 0.05 mm to 50 mm.
 17. The terahertzdifference-frequency generation quantum cascade laser source as recitedin claim 11, wherein a gap between each of said two or more mid-infraredfeedback gratings is etched into said upper cladding semiconductinglayer to electrically isolate and minimize crosstalk between each ofsaid two or more mid-infrared feedback gratings.
 18. The terahertzdifference-frequency generation quantum cascade laser source as recitedin claim 17, wherein said gap between each of said two or moremid-infrared feedback gratings has a length of approximately 5 μm to5,000 μm.
 19. The terahertz difference-frequency generation quantumcascade laser source as recited in claim 11, further comprising: tuningelements monolithically fabricated alongside said two or moremid-infrared feedback gratings or comprise external elements affixed toeach of said two or more mid-infrared feedback gratings, wherein saidtuning elements are electrically isolated from one another, wherein atemperature of each of said tuning elements is independently controlledwith a DC current, wherein said DC current applied to said tuningelements is independent of an electrical bias required to activate andquench said mid-infrared lasing.
 20. The terahertz difference-frequencygeneration quantum cascade laser source as recited in claim 11, furthercomprises an array of said quantum cascade lasers, wherein each of saidquantum cascade lasers is designed to emit and tune over a specificterahertz spectral range.